1. Field of the Invention
The present invention relates to a heat transfer sheet and a base sheet for a heat transfer sheet for transferring letters, patterns or the like on a substance to which they are transferred.
2. Description of the Prior Art
There have been used a heat transfer paper for transferring letters and figures and designs on a substance to which these letters, figures, patterns or designs and patterns are transferred (hereinafter referred to as "transfer substance") for the purpose of display and/or decoration. The heat transfer paper comprises substrates of paper or plastic films provided thereon with a thermally adherable transfer layer through a releasing layer or comprises the foregoing substrates provided thereon with a thermally transferable layer capable of being sublimated. When heat transfer paper is used for transferring letters, figures and designs to a transfer substance, there have been known a variety of transfer methods. For instance, an example thereof comprises printing desired letters, figures and designs, in advance, on a substrate with a thermally transferable material according to a proper printing method such as silk screen printing, gravure printing or offset printing and then transferring them to a transfer substance and another example thereof comprises applying a thermally transferable layer on the whole surface of a substrate, cutting out desired letters or patterns from the resulting assembly and then transferring it to a transfer substance.
A method in which letters, figures and designs are formed through printing is suitable for preparing a large amount of heat transfer materials of the same letters or figures and designs, but it requires too much expenses for printing and accordingly the unit cost of the products substantially increases in case of small scale production. On the other hand, in the method in which a heat transfer sheet comprising a thermally transferable layer applied onto the whole surface of a substrate is cut into desired letters or figures or designs and transferred to a transfer substance, desired letters or figures or designs can be formed in a predetermined amount according to need. To cut out letters or patterns from a heat transfer sheet can be performed by using a method in which they are cut out by handling. However, taking account of workability and uniformity of letters or patterns to be obtained, it is advantageous that an automatic cutting machine controlled by a computer is used for cutting out them from the heat transfer sheet. There have been known a variety of methods for preparing letters or patterns with such an automatic cutting machine. For instance, an example thereof comprises making grooves which reach the bottom of the substrate of the heat transfer sheet to cut out letters or patterns therefrom and then rearranging them and another example comprises making grooves only in the transfer layer to remove the unnecessary transfer layer. In the former method, the rearrangement of the letters or the patterns separately cut out from the transfer sheet is difficult and, therefore, the latter method is superior to the former.
In the latter method, the removal of the unnecessary portions of the transfer layer by peeling is easy when the thickness of the transfer layer which is applied onto a substrate through a releasing layer is thick, but it is difficult or impossible if the transfer layer is thin. In particular, the removal of the unnecessary portions is impossible in the heat transfer material provided with a thermally transferable layer capable of being sublimated.